The shaft of an armature core is typically press fitted into the central bore of the armature core of an electric motor. To ensure a rotationally fast engagement between the shaft and armature core, it is commonly practiced to form a plurality of longitudinal ridges on the outer circumferential surface of the shaft before press fitting the shaft into the central bore of the armature core.
According to a known method for forming such ridges on a shaft, a tool in the form of a metallic die is pressed upon the shaft between an upper and lower die assembly such that an edge of the metallic die forms a groove in the surface of the shaft by plastic deformation. This causes a plastic flow of the material of the shaft such that a ridge is formed on either side of the groove (See Japanese patent laid-open publication No. H05-200475).
However, this known method has the following disadvantages.    (1) A relatively large force has to be applied to the forming tools to produce ridges having a desired height, and the resulting residual stress is so great that the circularlity of the shaft may be impaired. Therefore, the centering precision in press fitting the shaft into the bore of a motor armature core or commutator cannot be made so high as desired.    (2) The tops of the produce ridges are relatively blunt, and there is almost no work hardening. Therefore, the engagement between the shaft and the bore of a motor armature core or commutator cannot be made so secure as desired.    (3) Because the axial length of each ridge is determined by the length of the tool, different tools are required for the different specifications of the motor shafts. Therefore, a large number of tools are needed when there are a large number of models and the production volume of each model is small. This results in a high manufacturing cost.    (4) A press of a large capacity typically in the order of 10-ton is required, and this requires a high initial cost.